The invention relates to techniques for making improved non-contact temperature measurements of a semiconductor or other substrate.
In many semiconductor device manufacturing processes, the required high levels of device performance, yield, and process repeatability can only be achieved if the temperature of a substrate (e.g., a semiconductor wafer) is tightly controlled during processing. To achieve that level of control, it is often necessary to measure the substrate temperature in real time and in situ, so that any unexpected temperature variations can be immediately detected and corrected for.
Consider, for example, rapid thermal processing (RTP), which is used for several different fabrication processes, including rapid thermal annealing (RTA), rapid thermal cleaning (RTC), rapid thermal chemical vapor deposition (RTCVD), rapid thermal oxidation (RTO), and rapid thermal nitridation (RTN). In the particular application of CMOS gate dielectric formation by RTO or RTN, thickness, growth temperature, and uniformity of the gate dielectrics are critical parameters that influence the overall device performance and fabrication yield. Currently, CMOS devices are being made with dielectric layers that are only 60-80 .ANG. thick and for which thickness uniformity must be held within .+-.2 angstroms (.ANG.). That level of uniformity requires that temperature variations across the substrate during high temperature processing not exceed a few degrees celsius (.degree. C.).
The wafer itself often cannot tolerate even small temperature differentials during high temperature processing. If the temperature difference is allowed to rise above 1-2.degree. C./cm at temperatures around 1000.degree. C., the resulting stress is likely to cause slip in the silicon crystal. The resulting slip planes will destroy any devices through which they pass. To achieve that level of temperature uniformity, reliable real-time, multi-point temperature measurements for closed-loop temperature control are necessary.
Optical pyrometry is being widely used for measuring temperatures in RTP systems. Pyrometry exploits a general property of objects, namely, that objects emit radiation with a particular spectral content and intensity that is characteristic of their temperature. Thus, by measuring the emitted radiation, the object's temperature can be determined. A pyrometer measures the emitted radiation intensity and performs the appropriate conversion to obtain temperature.
One difficulty encountered in the use of pyrometers for measuring substrate temperature in an RTP system is that variations between individual temperature sensors and differences in their position with respect to a particular substrate in the chamber can affect the accuracy of the temperature measurements. Therefore, substrate temperature measurements obtained from the sensors can have an unknown error component attributable to such variations. Those variations show up, for example, as differences in the thickness of a deposited layer across the substrate surface because the sensors are used as part of the closed-loop temperature control.
One way to address such errors is to reduce or increase a temperature offset at the location of the temperature deviation. If the location of the temperature deviation matches the position of a temperature probe, then the amount of temperature change necessary to correct the deviation is approximately proportional to the amount of deviation in the thickness of the deposited layer. Such a technique, however, assumes localized heating from the heat source. However, due to cross-coupling between lamp zones, thermal discontinuities at the substrate edge, and the viewing angle of the probes, such an assumption generally is not valid. Thus, additional techniques are required for obtaining accurate substrate temperature measurements to provide uniform processing conditions across the substrate surface.